Optical communications devices and methods employing same are a substantial and fast-growing constituent of communications networks. Applications of such methods and devices include, but are not limited to, telecommunications systems, cable television, and local area networks (LANs).
As the demand for such applications increases however, the current capacity of existing waveguiding media is limited. And while the capacity of the media may be expanded, e.g., by deploying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there has been a great demand for cost-effective methods and apparatus that increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) has been shown as a promising approach for increasing the capacity of existing fiber optic networks. A communications system employing WDM uses plural optical signal channels, each channel being assigned a particular channel wavelength. In such a WDM system, optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, transmitted over a single waveguide, and demultiplexed such that each channel wavelength is individually routed to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, plural channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance applications.
U.S. Pat. No. 5,631,758 issued to Knox et al. on May 20, 1997 disclosed a Chirped-Pulse Multiple Wavelength Telecommunications System. This patent, which is assigned to the assignee of the present application, is expressly incorporated herein by reference. As described therein, a chirped-pulse-WDM technique defines and encodes data on a large number of channels using a spectrally broadband source and a single modulator.
With this chirped-pulse WDM technique, laser pulses become chirped as they propagate through a dispersive delay line such as an optical fiber that introduces a time delay between their frequency components. Frequency bands that are useful as WDM channels are then selected from the pulses' continuous spectra through the action of a modulator operating at a multiple of the laser repetition rate, i.e., 36.7 MHz.
Proof of the effectiveness of the WDM technique was described by L. Boivin, M. C. Nuss, W. H. Knox and J. B. Stark in an article entitled "206-Channel Chirped-Pulse Wavelength Division Multiplexed Transmitter" that appeared in Electronics Letters, Vol.33, No. 10, pp. 827-828, 8.sup.th April, 1997 and incorporated herein by reference. As disclosed in this article and shown in schematic form in FIG. 1, a single femtosecond laser, a single dispersive optical fiber, and a single time-division-multiplexed electroabsorption modulator (EAM) is used to generate the 206 useful channels.
The single laser is used because combining and stabilizing a large number of single frequency lasers is prohibitively complicated and expensive. A mode-locked erbium doped fiber-ring laser is used as the source of sub-picosecond laser pulses with a bandwidth of &gt;70 nm and a repetition rate of 36.7 MHz. The spectrum of each pulse is mapped onto the time axis as it propagates through a single mode fiber that has a total dispersion of -340 psec/nm. This propagation stretches out the pulses to a duration of about 24.2 nsec and provides a nearly linear relationship between wavelength and time delay within each pulse.
A TDM electroabsorption modulator that has a 12-GHz bandwidth is installed at the output of the chirping fiber to define and encode data onto each channel in a time-sequential manner. It uses a TDM multiplexed pattern generator synchronized to the 271.sup.st harmonic, or 9.942 GHz of the laser repetition rate. The short-wavelength absorption edge of the modulator results in the spectral narrowing of the pulses to about 28 nm. To partially restore the original bandwidth and to equalize the transmitted spectrum, the modulator bias is adjusted dynamically using feed-forward equalization as each pulse passes through.
Additionally, when the frequency components of the short pulse laser are separated temporarily by propagation in a dispersive medium, a single TDM modulator can be used to encode data on all channels. Accordingly, a 271-bit TDM "word" is used for every pulse to encode data onto each of the 271 frequency slots defined by the modulator. When the same word is used for each pulse, a stable optical spectrum is obtained. The chirping fiber converts the TDM pattern into a WDM modulation with states "1" and "0" corresponding to high- and low-intensity frequency bands, respectively. Out of 271 possible wavelength slots defined by the modulator, 206 have been identified with sufficient optical power.
As can be readily appreciated, the larger the number of wavelength channels, the lower the speed at which related electronics have to operate thereby decreasing overall system cost. Also, the chirped-pulse WDM provides hundreds of WDM channels from a single source without the need to stabilize the wavelength of each channel. Consequently, there is a continuing need for an efficient and cost-effective WDM methods and apparatus that permit transmitting a large number of spectral channels.